Dynamic processing of dimethyl ether and methanol to gasoline (DMTG): Investigation of kinetic transitions at fluctuating workloads under isothermal conditions

Abstract

This is the first report on the transient behavior of ZSM-5 based catalysts in the conversion of dimethyl ether and methanol to hydrocarbons in the gasoline range (DMTG). With high temporal resolution, we analyzed the outlet flows after single/periodically repeated changes of feed concentration and volumetric inlet flow. Catalysts used were extruded cylindrical bodies of ɣ-Al2O3 and H-ZSM-5/ɣ-Al2O3 for the DME pre-reactor and the DMTG reactor, respectively. Experiments were carried out at residence times of 43–891 kg∙s∙m−3, a total pressure of 10 bar, molar feed fractions between 0.108 and 0.323, and DMTG reactor temperatures of 340–360 °C. Hypothetical outlet flows for infinitely fast kinetic transitions were calculated by combining the residence time characteristics of the reaction unit with steady state kinetics. Therefore, a new kinetic model was developed from experimental data. It is based on the dual cycle mechanism, considers the composition of the autocatalytic hydrocarbon pool and provides an excellent description of both the effluent compositions at all investigated process conditions and the deactivation of the catalyst. The comparison of calculated with the experimentally determined system responses on changes in workload enabled the detection and analysis of transient effects. Pronounced and long lasting transition states were observed after increases in the feed concentration. These transient effects were attributed to the enhancement of alkylation reactions and a concomitant shift in the hydrocarbon pool composition in favor of alkylated aromatics. Alkylated aromatics were found to play a key role for the activity, selectivity and deactivation behavior of the DMTG catalyst.